Mirror-ball for fisheye effect with orthographic camera
The sensor size or camera object scale has no effect on the projection. It works like this:



It's the camera plane (the base of the pyramid shape) projected into the scene.


render resolution sets the plane's sides ratio
orthographic scale sets the plane size, scale of 1.0 means the camera's plane is 1.0 units wide


You can calculate the camera corners with python:

Export rendered image corners coordinates

You will get them at camera origin where the sensor is, not where the camera border is displayed in viewport. Since the projection is orthogonal this should not matter.